The objective of this research program is to develop an understanding of the structural, electronic, and mechanistic features of hydroporphyrin-containing enzymes. These catalyze important steps in key metabolic processes including the inorganic carbon, nitrogen, and sulfur cycles and many of the main energy producing pathways of microorganisms. The program has three specific aims: (1) to determine the significant differences in the properties of the complexes of hydroporphyrins, porphyrins, and other tetrapyrroles, (2) to synthesize and characterize a structural model of the linked siroheme-Fe4S4 active site of nitrite and sulfite reductases, and (3) to develop a reactivity model for the F430 dependent reductive cleavage of S-methyl coenzyme M to methane. Synthetic, small molecule analogues of enzyme active sites and prosthetic groups will be used in these investigations. The first aim will be addressed by testing the hypotheses that porphyrin, hydroporphyrins, corrins, and other tetrapyrroles are distinguished by the optimal hole size and range of hole sizes readily accessible in their complexes and that the substituent stereochemistry can modulate the range of readily accessible hole sizes. The hypotheses will be tested by examining chemical changes that require large changes in metal ion radius. Ligand binding/spin state equilibria of Ni(II) tetrapyrroles and the correlation between the accessibility of the Ni(I) oxidation state and the saturation level and substituent stereochemistry of the hydroporphyrin macrocycle will be investigated, among others. The third aim will be achieved by exploring the reactions of NiI(OEiBC)-, the only reported Ni(I) tetrapyrrole except for F430. The mechanism(s) of the reactions will be determined. Alkyl- Ni(OEiBC) complexes will be synthesized and evaluated as potential reaction intermediates.